The present invention relates to improving the performance of aromatic polyimide and polyamide-imide membranes used for the separation of mixtures of gases. Aromatic polyimide and polyamide-imide gas separation membranes are known in the art. Such membranes generally fall within one of two categories. One such category incorporates membranes with high relative selectivities for one gas over another gas permeating through the membrane. Such membranes, however, suffer from having low gas permeation rates. Another category of polyimide gas separation membranes have much higher gas permeation rates, but suffer from correspondingly lower relative gas selectivities. Polyimide gas separation membranes made from aliphatic diamines have not been useful due to poor gas permeation properties and difficulties in polymer synthesis.
It is desirable to have polyimide gas separation membranes which exhibit high gas permeation rates while maintaining high relative gas selectivities. Further, it is desirable for such materials to be easily fabricated into appropriate membrane structures while maintaining excellent end use environmental resistance.
Prior art references to polyimide and polyamide-imide gas separation membranes are abundant. For example, U.S. Pat. No. 3,533,997 discloses aromatic polyimides which incorporate a pendant carboxylic acid function and the crosslinking of such materials through interaction of the pendant carboxylic acid functions and di- to tetra-amine radicals. The polyimides of the present invention do not contain such pendant carboxylic acid functions.
U.S. Pat. No. 4,734,464 discloses a solvent resistant composition comprising a siloxane-containing polyimide and an aromatic compound containing at least two reactive groups such as amino groups which are heated to at least 150.degree. C. The present invention does not incorporate siloxane-containing polyimides or the high temperature treatment.
The prior art does not describe or teach a polyimide or polyamide-imide gas separation membrane derived from an esterified polymer which is made from a blend of amines comprising 1-75% of the ester derived from 1,3-diamino-2-hydroxypropane ("DAHP").